Target and method of optimizing target profile

ABSTRACT

A method of constructing increased life sputter targets and targets made by the method are disclosed. The method comprises starting with a precursor target design or profile and making magnetic field strength measurements along the radial surface of same and at a plurality of vertical dimensions above the surface. An optimal magnetic field strength ratio is provided between the erosion tracks of the target. The vertical dimension of the material to be added to one of the erosion tracks is determined and then the height of the other erosion track is calculated by utilizing this optimal magnetic field strength ratio.

CROSS-REFERENCE TO RELATED APPLICATIONS

Priority filing benefit of (1) International PCT applicationPCT/US02/04819 filed Feb. 20, 2002, and published under PCT 21(2) in theEnglish language; (2) U.S. provisional application Ser. No. 60/286,182filed Apr. 24, 2001; (3) U.S. provisional application Ser. No.60/296,354 filed Jun. 6, 2001; (4) U.S. provisional application Ser. No.60/300,019 filed Jun. 21, 2001; and (5) U.S. provisional applicationSer. No. 60/328,847 filed Oct. 11, 2001.

BACKGROUND OF THE INVENTION

In a commercial physical vapor deposition (PVD) system, the lifetime ofa sputtering target is usually designed by the original equipmentmanufacturer (OEM). Such a lifetime, usually defined as a sputteringpower times sputtering time (kilowatt-hours) or as the total thicknessof material deposited on the substrates (microns, or number of 1 microndepositions), is mainly determined by the sputtering target material,target geometry and cathode magnet design. The sputtering cathodeassembly is optimized for its performance parameters, such as depositionuniformity for the given design.

Target uniformity performance is determined mainly by the followingthree factors: the erosion profiles of the target during sputtering,target crystallographic texture, substrate to target distance, and thegas scattering factor during the deposition process. The erosion profileis the most important factor in determining the deposition uniformity.It also remains largely unchanged for a given PVD system.

In a PVD process, a cloud of plasma is present in front of thesputtering target. This plasma is sustained by the magnetic field fromthe magnets behind the sputtering target. The density of the plasma and,hence, the rate of sputtering of the target is related to the magneticfield strength at the target surface. Electrical-magnetic theoryindicates that the maximum sputtering rate occurs when the verticalcomponent of the magnetic field is zero and the horizontal component ofthe magnetic field is at maximum. In the following, the term “magneticfield” refers to the horizontal component of the magnetic field when thevertical magnetic field is near zero if it is not otherwise indicated.

In an advanced magnetron PVD design, the cathode magnet usually consistsof an array of small magnets rotating around a target center axis togive better uniformity performance. At different locations on the targetsurface the magnetic field strength and the average residence time ofthe magnetic field per revolution of the magnets vary. Both of thesevariations contribute to the existence of different sputtering rates atdifferent locations on the target surface, hence the existence of thetarget sputtering profile (sputtering grooves). We define the timeintegration of the magnetic field strength within a revolution as timeaveraged magnetic field strength (T-B-Field). In a commercial PVDsystem, the OEM usually designs the configuration of the cathode magnetassembly to form the desired T-B-Field. This, in turn, creates thedesired target surface erosion profile that is adapted to achieveoptimal deposition uniformity performance. Methods for determiningdesired magnet configuration and target erosion profiles may be seenupon review of U.S. Pat. Nos. 4,995,958; 5,314,597; 5,248,402;5,830,327; and 5,252,194.

DESCRIPTION OF THE INVENTION

There are situations where a longer target utilization lifetime isdesired. The simplest way to try to accomplish this is to increase thethickness of the sputtering target. However, since the cathode assemblyis optimized for the designed thickness, an increase of thickness mightcause the deterioration of the deposition uniformity. In a recent test,a 13% increase of target thickness caused the target depositionuniformity to change from 0.7% to 1.18% at 1 σ. The thin film resistancecontour map (Omni-map) shows that less material was sputtered from thecenter of the target as compared to the outer edge of the target, i.e.,the sputtered thin film was thinner at the wafer center than at thewafer edge. We discovered that this change was due to the fact that theT-B-Field at the center and at the edge no longer held their properratio when the target thickness was increased. In order to sustain thedeposition uniformity performance, we found that the target thickness atdifferent erosion groove locations had to be changed in order to bringthe local T-B-Field back to the proper ratio.

The method below describes how to find the proper profile (shape) of anincreased thickness target in order to achieve the goal of increasingtarget life while maintaining target uniformity performance. For anygiven sputtering target configuration, our method involves the followingsteps:

A) Measuring the existing target sputtering profile and determining themaximum erosion groove locations.

B) Measuring the vertical and horizontal magnetic field strength atdifferent radial locations of the target surface. Since the magneticassembly is rotating, this measurement has to be carried outdynamically. This can be accomplished by using two or three B-fieldprobes to measure the magnetic field strength at any location on thetarget surface at three different orthogonal directions simultaneously.The results of the B-field probes can then be fed to a digitaloscilloscope, and the results of the dynamic magnetic field strength atdifferent directions can then be calculated at any location of thetarget surface. Additional measurements are taken at different radiallocations on the target surface.

C) Continuing the same measurements as in B) at different heights abovethe target surface. The measurements should exceed the height equal tothe intended increase of target thickness.

D) Recording or plotting the graph of the T-B-Field vs. radial locationsat different heights above the target surface. (This step could becompleted in a computer or other memory means.)

E) Determining the ratio of the T-B-Field at each erosion groovelocation relative to the location of the deepest groove at the targetsurface level.

F) Determining the ratio of the T-B-Field at each erosion groovelocation to the location of the deepest groove at the height equal tothe increased target thickness level.

G) Determine the change needed of the height at each groove location sothat the ratio in F) will match the ratio in E).

From the difference of local height and width of each groove obtained inG), the extended life target profile can then be designed.

The invention will be further described in conjunction with the appendeddrawings and remaining description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a dual erosion track target/backingplate assembly that can be designed in accordance with the invention;

FIG. 2 is a graph showing time integrated magnetic field strengths vs.increased vertical distance or increased height of the target materialcompared to a standard target profile;

FIG. 3 is an erosion profile calculated by the methods reported in theexample set forth in the description;

FIG. 4 is a sputtering uniformity data graph showing sputteringuniformity and increased sputter energy lives (in kwhr) for increasedthickness targets designed and made in accordance with the invention;and

FIG. 5 is a cross-sectional view of a prior art dual erosion tracktarget/backing plate assembly.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Turning now to FIG. 1 there is shown an exemplary target that may beenhanced by the instant method so as to result in a greater targetthickness to increase sputtering life without deleteriously affectingsputtering performance. The particular target shown is adapted for atype “B” ENDURA® sputtering system.

Target 12 may, for example, comprise Al with backing plate 14 comprisingAl or Cu. Alloys of these metals may also be noted as exemplarymaterials. Most preferably, the target 12 is bonded to the backing plate14 in accordance with the disclosure of commonly-assigned PCTInternational Application WO 00/15863, its corresponding U.S. patentapplication Ser. No. 09/720,347, filed Dec. 21, 2000, the disclosures ofwhich are incorporated herein by reference.

The preferred target 12 is generally frusto-conical in shape, beingcircular in plan and possessing side walls 20 which converge in agenerally linear fashion in the direction of a sputtering surface 22. Incross-section, the preferred target 12 and backing plate 14 have theoverall configuration of a frustum, with the backing plate 14 serving asthe base of the cone and the target side walls 20 serving as themid-position of the cone, such that the side walls 20 would approach anapex of the cone if the side walls 20 were extended beyond thesputtering surface 22.

A thickened area or circular boss 30 is formed along the target/backingplate interface 32. This thickened area 30 serves to increase targetlife by acting as an erosion track extension or the like. In theembodiment illustrated, the radial dimension of this thickened area 30is preferably about 3.047 inch (˜{fraction (7/74)} cm) and the depth ispreferably about 0.050 inch (˜1.3 mm).

The sputtering surface 22 includes an outer, stepped-up or elevatedterrace area 40 of increased target thickness surrounding a shallow well42 defining a thinner, central region of the target 12. The terrace 40comprises an outer wall 50 and an inner wall 52. The inner wall 52slopes outwardly from the well toward a plateau or outer surface 54 ofthe terrace at an angle of about 13.5° in the particular embodiment thatis depicted. The inner wall 52 has a length or radial dimension of about0.25 inch (˜6.4 mm). As shown, the surface of the terrace 40 is raisedabout 0.060 inch from the surface of the well. The terrace 40 providesadditional material thickness in a region of the sputtering surface 22where high erosion can be anticipated.

The target shown in FIG. 1 is known as a dual erosion track targetwherein the target comprises an outer erosion track 102 and innererosion track 104 shown in phantom in FIG. 1 (as can also be seen in theprofile shown in FIG. 3) with these tracks disposed generally in anannulus around the target. In the view shown in FIG. 1, the outererosion track is defined as the vertical distance between the topsurface of the terrace 40 and bottom of the thickened portion 30 of thetarget with the inner erosion track contained in the region of thetarget circumscribed by well 42 and having a vertical distance measuredfrom the bottom of the target to the top of the well.

In general, the methods of the instant application can be employed toprovide enhanced sputtering life to the target by providing optimizedincrease of the height of the erosion tracks.

From the data now available to us it appears that there are a fewparameters that limit the vertical or height extension that can be madeto existing standard targets. Initially, in order to promote effectivesputtering, the time averaged magnetic field strength must approximateabout 200 gauss at the target surface area so that the desired plasmaconfiguration can be formed along the target surface. Additionally, thephysical spatial limits of the sputtering system provide an upper limitto the desired height increase of the target.

Lastly, and most importantly, the sputtering uniformity of the increasedheight targets must be similar or better at a 1 σ confidence level tothe uniformity achieved by the present standard height targets. That is,a uniformity of less than about 1% at 1 σ should be achieved.

It is to be noted that the principles of the instant invention arethought applicable to any type of PVD system where the plasmaconfiguration is magnetically controlled and which results in a targeterosion profile having two or more erosion tracks. For instance, Endura®systems available from Applied Materials, Quantum™ systems from Novellusand Ulvac® systems from Ulvac® may all be mentioned as exemplary.

The goal then is to take a standard target or predicted target profilethat presumably already has a desired or optimized sputter trackconfiguration and known vertical height dimensions for the erosiontracks. From the precursor profile we can then confidently increase theheight of the erosion tracks to make the target thicker withoutdiminishing sputter performance.

Turning now to FIG. 2, there is shown a graph plotting the B-Field TimeIntegration (T-B-Field) values in Gauss-sec for vertical spaces over thestandard target in those radial locations which correspond to the outererosion track and inner erosion track. It can be seen that for thestandard target, designated as 0 mm, the field strength measured at theouter erosion track of the purportedly optimized target is 1030gauss-sec and the magnetic field strength at the inner erosion track is940 gauss-sec.

Thus, in accordance with the invention, it will be desirable to maintainthis ratio of outer erosion track magnetic field density (OETGS)/innererosion track magnetic field density (IETGS), i.e., (OETGS):(IETGS), atan approximate constant ratio.

Here, for the particular system tested, the most preferred OETGS/IETGSratio is 1.095. This is seen by comparing the OETGS and IETGS valuesalong the y-axis with x=0. Thus, a preferred range for OETGS/IETGS ratiofor this particular system would be about 1.00-1.20:1.

If the artisan then desires to increase the thickness of the outererosion track area by 6.60 mm as per FIG. 2, the anticipated OETGS willbe 817 gauss-sec. If the optimum OETGS/IETGS ratio of 1.095 is to bemaintained, then the IETGS must be 746 gauss-sec. This means that theheight of the inner erosion track must be increased by 5.35 mm leaving a1.25 mm distance between the two erosion tracks. This would correspondto a 1.25 mm difference between the height of the terrace 40 shown inFIG. 1 and the height of the well 42.

Stated differently, for the two groove target shown in FIG. 1, theT-B-Field at the inner and outer groove locations are measured atdifferent heights. For the desired increase of 6.60 mm of targetthickness, a 0.05″ deep pocket has to be cut at the location of theinner groove in order to maintain the same T-B-Field ratio as in thestandard target or the purported optional profile configuration. Thisincreased height target was tested in an Endura® sputtering system andresulted in a 100% increase in sputter life with the same depositionuniformity demonstrated by the standard target. Results can be seen inFIG. 4.

The following example demonstrates one method by which a desired targetsputtering profile may be generated. It is to be understood that thisexample illustrates only one method by which a desired target sputteringprofile may be generated. Additional methods involve using an existingtarget that purportedly has optimal erosion track dimensions andconfiguration. Also, a multiplicity of target configurations canactually be test sputtered so that by trial and error a specificconfiguration can be obtained that is thought to be optimum. This targetdesign then is the starting configuration or precursor configurationthat is optimized in accordance with the invention by increasing thevertical dimension or height of the target material in the erosiontracks. Additionally, a variety of mathematical formulae may be used topredict erosion track configurations such as those set forth in theprior art U.S. patents listed above.

EXAMPLE ONE

(A) A target adapted for use in the Quantum™ sputtering system wasmounted with target surface up. The magnets were rotated as duringsputtering. Two Hall-Effect Gauss Probes were mounted on a slidingmicrometer to cover all radial locations of the target. Anothermicrometer was used to adjust the height of the probes to cover all fiveheight positions used in this study. The Hall Effect Gauss probe couldthen measure the magnetic field of the Quantum™ source in Radial (R),Tangential (T) and Vertical (Z) directions.

(B) We used two Gauss Probes to measure simultaneously the dynamicmagnetic field strength of the Quantum™ source in Radial (R)/Tangential(T) and Radial (R)/Vertical (Z) directions. Because of the low frequencyof the signal (˜1 HZ), the two pairs of measurements were notsynchronized. However, within a measurement, the two channels weresynchronized. We measured the magnetic fields at 30 different radiallocations and at 5 different heights from the target surface. At eachlocation there are four channels of data (R, T and R, Z) and eachchannel contains 40,000 data points. All of the two channel wave formswere displayed in a two-channel Tektronix Oscilloscope and thesynchronized two-channel data were exported to a PC.

(C) For each radial location and each height, the recorded four channelsof data were inputted into an Excel® Spread sheet. Then the data werecondensed by factor of 10 by simply averaging every 10 data points intoone data point. A conversion factor was also incorporated to compensatefor the different magnification for each channel. Then the four channelsof condensed data were stored into a new data set.

(D) For each new data set, the two R data series were plotted in Excel®and the specific time shift was determined for each two pairs of dataseries. Then the R/Z data group was shifted by the correct amount oftime to merge together with the R/T group. Now the R, T and Z serieswere all synchronized. The R and T channel data were then vector-addedto form the parallel B-field (B-p) strength. A new file was created forthis synchronized B-p and Z data series.

(E) For each B-p and Z series, each B-field value was time-gated asfollowing: (1) The absolute value of the Z field must be less than apreset parameter (150 Gauss in this report) and, (2) The B-p value mustbe more than another preset parameter (50 Gauss is this study). Thegated B-field strengths were then integrated within a revolution periodto obtain the relative Time-Integration Average for the parallel B-fieldstrength when the vertical B-field were near zero (T-B-Field).

(F) For the first order approximation, this T-B-Field result should beproportional to the sputtering rate at that location. These results wereplotted and scaled against a real measured Erosion Profile. This graphis shown in FIG. 3. The reference numerals 102, 102 show what has beenreferred to as the outer erosion track with the inner erosion trackgiven reference number 104, 104.

The instant method may be utilized to construct plural or even multipleerosion track magnetron sputtering targets. The first step is to providea desired precursor sputtering target configuration having what isthought to be optimal profile for uniform sputtering performance. Thephrase “sputtering target configuration” should be construed to covernot only existing targets themselves that are thought to have optimizedor have standard construction, but also computer generated ormathematically derived profiles or configurations. The precursorsputtering target configuration may, for example, be obtained frominspection of existing sputtering targets or, such profiles can bedetermined via actual sputtering of a plurality of sputtering targetsamples with the optimal profile then being decided by trial and error.Once the precursor sputtering target configuration is obtained, thelocation of the erosion tracks may be determined.

In accordance with one aspect of the invention, the desired precursorsputtering target configuration is obtained by making magnetic fieldstrength measurements at a variety of radial locations along thesputtering surface and by making the same magnetic field strengthmeasurements at a plurality of vertical dimensions above those radiallocations. The erosion tracks are correlated to those measurements.

Once a desired precursor sputtering target configuration has beenobtained, magnetic field strength measurements are made at a variety ofdifferent radial locations RL along the sputtering surface of theprecursor sputtering target profile. Due to the rotation of themagnetron assembly, a time-averaged magnetic field strength is obtainedfor the radial points on the target surface located at the erosion tracklocations.

An optimum ratio range for the magnetic field strengths as measured atthese radial locations corresponding to the erosion tracks is thendetermined; namely, OETGS/IETGS. This optimum ratio range will, ofcourse, vary from sputtering system to sputtering system. We have foundthat for the particular Endura® system, the outer to inner erosion trackratio should be on the order of about 1.00-1.20:1. The preferredmagnetic field strength ratio of the outer to inner erosion track inthis system is 1.095:1.

Magnetic field strength data points are then obtained for a variety ofvertical dimensions V located above the different radial locations RLover the erosion tracks E-1 and E-2. These data points are identified asVE-1 and VE-2.

All of this data may then be recorded in a memory media such as acomputer or on graph papers, etc. The data points may, for example, beplotted on a graph with one axis of the graph including the magneticfield strengths and the other axis reporting the VE-1 and VE-2 verticalheight dimensions at which such magnetic field strengths were measured.

Then, the desired increase in vertical dimension for the sputteringmaterial to be located at one of the erosion tracks is determined. Themagnetic field strength for such increased vertical dimension is thendetermined and the magnetic field strength for the other erosion trackis then determined utilizing the optimal ratio range for magnetic fieldstrengths existing at erosion track locations RE-1 and RE-2. Once thisvalue has been obtained, the artisan can then determine the desiredincrease in vertical dimension or height for the sputtering material tobe applied or added to the other erosion track. Of course, after thedetermination of the vertical extension for each of the erosion tracksis determined, a sputter target in accordance with those dimensions isconstructed.

Turning now to FIG. 5, there is shown a prior art or standardtarget/backing plate assembly. The thickness of the target in thisassembly can be optimally increased in accordance with the inventionwhile the erosion track heights are optimally balanced by use of themagnetic field strength ratio method above described; all resulting inan enhanced life, uniformly sputtering target of the type shown in FIG.1.

The prior art assembly may comprise a distinct target structure bondedover a backing plate or the assembly can be monolithic. As shown, theprior art assembly comprises target 112 bonded over backing plate 114.In conventional manner, the lower side 115 of the backing plate isadapted for heat exchange contact with a cooling medium, typicallywater.

Note that in the FIG. 1 target, no recess or well area is formed alongthe top 117 or sputtering surface of the target. Two distinct annularerosion tracks 202, 204 do exist in the target, due primarily to therevolution of a specifically designed magnet array under the surface115.

The overall height (h) of the assembly of FIG. 1 including the backingplate (and its associated mounting flange 130) and the target is 1.9″.The thickness of the target itself (i.e., vertical distance from backingplate to the top of the sputtering surface) is 0.772″ with the backingplate thus having a height of 1.128″.

In accordance with the invention, and with respect to the FIG. 1assembly, we have constructed an effective target/backing plate assemblyhaving an overall height of 2.16″, whereby the target thickness (asmeasured from the target/backing plate interface 32 to the top of thesurface of terrace 40) is 1.032″. The backing plate thickness here is1.128″ same as for the standard target. Thus, the difference in targetthickness between the specific embodiment of the invention shown hereinin FIG. 1 and the standard or FIG. 5 target is 0.26″ (or 6.6 mm).

The method described previously is then used to calculate the thicknessneeded for the inner erosion track (note FIG. 2 herein). By this method,a recess of 1.25 mm is calculated and this recess is formed by machiningthe target surface to form the recess or well 42.

Both of the targets shown in FIGS. 1 and 5 have a diameter of 15.620″with the sputtering surface of each of the targets having a diameter of12.394″.

The sputter targets in accordance with the invention thus havethicknesses greater than the standard thickness of 0.772″. Additionally,these targets in accordance with the invention are characterized byhaving time-averaged magnetic field strength ratios, as previouslydiscussed, of RE-1:RE-2 of from about 1.00-about 1.20:1. Morespecifically, and as set forth in the example above, the RE-1:RE-2 ratioof 1.095:1 is presently preferred in conjunction with targets adaptedfor utilization in conjunction with the Endura® sputtering systems.

Turning back to FIG. 1, the thickness of the terrace area of the targetin this embodiment is about 1.032″ with the thickness of the well 42 ofthe target shown equaling about 0.972″. Accordingly, the well isrecessed from the terrace in an amount of 0.060″. However, the artisanwill appreciate that the well surface could be recessed from the terraceby a dimension of about 0.04-0.075″, more preferably in accordance withthe above method by about 0.05-about 0.06″.

The thickness of the entire assembly of backing plate and target ormonolithic assembly is greater than about 1.9″ in accordance with theinvention and the erosion tracks E-1 and E-2 located on the target willexhibit time-averaged magnetic field strengths of RE-1:RE-2 of fromabout 1.00-about 1.20:1.

While the form of apparatus herein described constitutes a preferredembodiment of this invention, it is to be understood that the inventionis not limited to this precise form of apparatus, and that changes maybe made therein without departing from the scope of the invention whichis defined in the appended claims.

1. A method of determining a desired thickness of plural erosion tracksin a magnetron sputtering target comprising: a) providing a desiredprecursor sputtering target configuration having a specified sputteringsurface and determining the location of said erosion tracks, E-1 andE-2, thereon; b) determining the magnetic field strength at differentradial locations RL along the sputtering surface of said precursorsputtering target configuration and determining a time averaged magneticfield strength for points on said erosion tracks, RE-1 and RE-2; c)determining an optimum ratio range for said magnetic field strengthsRE-1 and RE-2 determined in (b); d) determining magnetic field strengthsat vertical dimensions V above said erosion tracks E1 and E2 such thatdata points VE1 and VE2 are generated; e) recording the magnetic fieldstrengths for said vertical dimensions VE1 and VE2; f) determining adesired increase in vertical dimension for sputtering material at one ofsaid erosion tracks E1 or E2; and then g) determining the desiredvertical height for sputtering material to be added to said sputteringsurface at said other erosion track.
 2. A method as recited in claim 1wherein said step e) comprises plotting a graph showing magnetic fieldstrengths for a plurality of vertical dimensions.
 3. A method as recitedin claim 1 wherein said step e) comprises recording data for saidmagnetic field strengths for said vertical dimensions VE1 and VE2 in amemory media.
 4. A method as recited in claim 3 wherein said memorymedia comprises a computer.
 5. A method as recited in claim 1 whereinsaid step a) comprises providing a sample sputtering target to serve assaid precursor sputtering target configuration.
 6. A method as recitedin claim 1 wherein said step a) comprises determining said precursorsputtering target configuration by measuring the magnetic fieldstrengths above a sample sputtering target at a plurality of radiallocations along the sputtering surface of said target and at a pluralityof vertical dimensions above the target surface.
 7. A method as recitedin claim 1 wherein said step g) comprises utilizing said optimum ratiorange determined in said step c) and calculating the desired magneticfield intensity for said other erosion track.
 8. A method as recited inclaim 1 wherein said optimum ratio range for said magnetic fieldstrengths at erosion tracks RE-1 and RE-2 is about 1.00-1.200:1RE-1:RE-2.
 9. A method as recited in claim 8 wherein said magnetic fieldstrength ratio RE-1:RE-2 is about 1.095:1.
 10. A method as recited inclaim 1 wherein said magnetron sputtering target comprises amultiplicity of erosion tracks.
 11. Sputter target having a sputteringsurface with erosion tracks E-1 and E-2 located thereon and having asputter target thickness of h; wherein h>0.772″, said target having atime averaged magnetic field strength of RE-1 for points on said erosiontrack E-1 and a time averaged magnetic field strength of RE-2 for pointson said erosion track E-2 such that RE-1:RE-2 is from about 1.00-about1.20:1.
 12. Sputter target as recited in claim 11 wherein RE-1:RE-2 isabout 1.095:1.
 13. Sputter target as recited in claim 11 wherein saidE-1 and E-2 each have a thickness of TE-1 and TE-2, respectively,wherein TE-1 is about 1.032″ and TE-2 is about 0.972″.
 14. Sputtertarget assembly having a sputtering surface and a backing plate surfaceopposed from said sputtering surface and adapted for contact with a heatexchange medium, said assembly having a thickness of greater than about1.9″, said sputtering surface including erosion tracks E-1 and E-2located thereon, said target having a time averaged magnetic fieldstrength of RE-1 for points on said erosion track E-1 and a timeaveraged magnetic field strength of RE-2 for points on said erosiontrack E-2 such that RE-1:RE-2 is from about 1.00-about 1.20:1. 15.Sputter target as recited in claim 14 wherein RE-1:RE-2 is about1.095:1.
 16. A sputter target comprising a body of generallyfrusto-conical form including a sputtering surface, said sputteringsurface including a thickened annular terrace area surrounding a well,said well having a surface recessed from said terrace by about0.04-0.075″, wherein said sputtering surface comprises a time averagedmagnetic field strength of RE-1 for points on said terrace and a timeaverage magnetic field strength of RE-2 for points of said well suchthat RE-1:RE-2 is from about 1.00-1.20:1.
 17. A sputter target asrecited in claim 16 wherein said well surface is recessed from saidterrace by about 0.05-about 0.06″.
 18. Sputter target as recited inclaim 16 wherein said terrrace has a thickness of greater than about0.772″.
 19. A sputter target as recited in claim 16 wherein RE-1:RE-2 isabout 1.095:1.